The guess correction is based on a formula by Cohen-Schotanus (rewritten by W. Bender in 2003). The guess correction is only applicable to multiple choice questions with one correct answer and not for questions with multiple correct answers.
Corrected score = [(s-n*g) / (n-n*g)] x n
- s = score for multiple choice questions of the student
- n = maximum score for multiple choice questions
- g = (number of correct answers for multiple choice questions) / (total number of multiple choice answer possibilities)
The formula accounts for the possibility that multiple answers are correct for a multiple choice question by adding the total number of correct answers for multiple choice questions and dividing it by the total number of multiple choice answer possibilities. This way, for one multiple choice question with 4 answer possibilities of which 1 is correct, the guess correction factor g is equal to 1/4 (25%).
When more than one multiple choice question is in one assignment, the number of correct answers of all multiple choice questions are also added as the total number of answer possibilities of all multiple choice questions. The same is also done for the score s as well as the maximum score n of all multiple choice questions.